How to easily make mesh retopology?
Here are some wonderful tools for speeding up the retopology process:


polystrips (https://www.youtube.com/watch?v=VN1aC7DsS6A)
contours (https://www.youtube.com/watch?v=5RR06wjt2rs)


These are paid, but worth every penny as they're really time-saving tools.

If you don't want to buy them you may do a fast retopology using a Bsurfaces addon. Go to User Preferences (Ctrl+Alt+U-->Add-ons) and enable it.


Add an object you want to retopologize, then add a Plane (or any other object you like), enter the Edit Mode and delete it (x). In the Properties Panel (N) add a new Grease Pencil layer, then go to the Tool Shelf (T)-->Grease Pencil and change a stroke placement to Surface.


Now go to the Tool bookmark in a Tool Shelf. Draw some strokes on an object (hold D, then press LMB [ How to draw grease pencil on a surface (blender 2,75)? ]). When finished, click on Add Surface in Bsurface panel. Change the settings below as you like.